Patch Testing

St John’s Institute of Dermatology

Patch testing

Patch testing is an investigation for finding out if you have a specific allergy to a chemical substance that has been in contact with your skin. These chemicals include those found in cosmetics, skincare products, toiletries, and medical lotions or creams, in the home, at work and the general environment. It can tell us if eczema (dermatitis) is caused, or aggravated, by contact allergy to chemicals. Patch testing is a standard procedure that might cause some inconvenience but discomfort is usually minimal.

Small chambers containing the substances we want to test (possible allergens) are applied to your back with adhesive tape. Your back is usually used as it is convenient, but sometimes the tests might be on the upper arms or somewhere else. This might be because your back has active eczema at the time of testing, or because all the test chambers cannot be fitted onto your upper back.

We normally use IQ Ultra™ chambers with allergens from Chemotechnique Diagnostics™. Use of other test chamber types and allergens from other suppliers are sometimes necessary.

The chambers stay in place for 2 days before being removed. We take readings (observations) 2 times over the next week to note any reactions.

Lots of people have a contact allergy that causes no problems. If we identify a specific contact allergy, it does not mean that the allergy is the cause of your current skin condition.

Patch testing does not detect allergies that might be related to diet, asthma or hay fever. Skin prick tests and detection of specific IgE (immunoglobulin E) antibodies in the blood are used for this type of allergy.

What substances will you be tested with? All patients are tested with a series of contact allergens, to detect the most common ones. You might also be tested with other allergens which we think might be relevant to you. For example, allergens in cosmetics, perfumes, footwear, or clothing depending on your history, exposure scenarios, products used, and the site of your eczema.

1 of 4 What to bring to the clinic Bring any products you think might aggravate your skin, as well as those you use regularly such as toiletries, cosmetics and medical lotions or creams. We will need to take a small amount of them to do the test. If you cannot bring samples, please bring a list.

If you think a substance at your workplace might be aggravating your skin, bring a small sample and, if possible, the material safety data sheet (this should always be available from the health and safety, or HR departments).

Pre-patch test advice It is best to wear an old shirt, vest, blouse or t-shirt during patch testing as a small amount of the test substances, skin marking ink and petrolatum (used to dilute the allergens) might stain.

Do not apply any creams or moisturisers to your back for a day before the tests.

If your back is very hairy, please shave or clip the hairs away at least a week before your first appointment.

If you are on prednisolone tablets (more than 15mg each day) or medicines that suppress the immune system (such as methotrexate, azathioprine or ciclosporin), please contact us before your appointment in case the tests need to be delayed

Ultraviolet (UV) light (from sunlight), can suppress a patch test reaction. Because of this, we do not patch test on tanned skin, and recommend no UV light exposure from the sun or artificial sunlight (sunbeds) for 6 weeks before the test.

Dos and Don’ts Don’t  get your back wet during the week you are tested. You can sit in the bath and shower the front but your back must be kept dry.  participate in activities that might dislodge the tests by excessive sweating or stretching.  apply cream or ointment to your back, as the patches will slide off.  wear a new or valuable shirt, vest, blouse or bra, as it might get damaged.

Do  contact us with any problems.  remove the tests if they cause unbearable itching or discomfort.  apply extra tape if the tests appear to becoming loose.  wear night clothes to protect the tests.

Please note  You can continue to take antihistamines during the tests.  We do not perform patch testing on pregnant women unless absolutely necessary.  Children can be patch tested, but for practical reasons, a restricted range of test substances might be used.

2 of 4 What happens? Most people will need 3 visits.

At the first appointment your doctor will ask about your history and determine which substances you need to be tested for.

The tests will then be prepared by us for you. This appointment can last up to 2 hours.

The filled test chambers are applied to your skin by an experienced nurse who will also talk through the procedure with you and answer any questions.

A photograph is taken of your back with your own phone (if you have one), if you consent, to record the test application site.

The test chambers are removed 2 days after application. This might be done by you or in the clinic. The marking tape with the numbers should be left in place until your final reading. You will be given instructions on how to do this.

The second appointment is on day 2 or 3 after the chambers have been applied. The first reading of any reactions is made in the clinic. If there is a doubtful reaction, a test might need to be repeated.

The third appointment is on day 4 or 5 after the chambers have been applied. The second reading of any reactions is made. This will help confirm if the first readings are true allergic reactions (still present) or identify new reactions (not seen at day 2 or 3). Sometimes a third reading is necessary, but this will be discussed with you at the time of your last appointment if we need one.

During your last appointment your doctor will discuss the results with you and explain the results.

After your last consultation, you can bathe or shower normally. Any staining of the skin will go within a few days. Any positive reactions will normally settle over a few days but can be longer. A steroid cream can be used to speed this up.

A letter will be sent to the doctor who referred you, your GP, and you with details of the test results.

Complications Any positive reaction is usually no more than a 1cm square red patch. However, very allergic people might get small blisters (vesicles) or spread of the reaction to the surrounding skin. If this happens, a strong steroid cream will be applied to settle the reaction (an acute dermatitis).

Very rarely, a delayed reaction might develop a few days after the tests. If this happens, please let us know.

3 of 4 Pigmentary changes (lightening or darkening of the skin) is rare but can happen at the site of a marked reaction in darker skin types. Any changes will settle in time.

Results After the test, we will be able to confirm if any of the tested products are related to your current eczematous skin condition.

Contact us If you have any questions or concerns about patch testing, please contact the cutaneous allergy team, tel: 020 7188 , Monday to Friday, 9am to 5pm.

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